The Diencephalon is composed
of two structures: the thalamus and the
hypothalamus.

The thalamus is the large, two-lobed
structure that constitutes the top of the brain stem. One lobe sits on
each side of the third ventricle, and the two lobes are joined by the
massa intermedia, which runs through the ventricle.
Visible on the surface
of the thalamus are white lamina (layers) that are composed of myelinated
axons. The thalamus comprises many different pairs of nuclei, most of which
project to the cortex. Some are sensory relay nuclei  nuclei that
receive
signals from sensory receptors, process them, and then transmit them to
the appropriate areas of sensory cortex. For example, the lateral
geniculate nuclei,
the medial geniculate nuclei, and the ventral posterior
nuclei are important relay stations in the visual, auditory, and
somatosensory
systems, respectively. The thalamus seems to be a relay for sensory input
as well as an important part of other pathways, including motor and sensory
pathways and those between different parts of the cortex and the cerebellum
and other subcortical structures. The thalamus and the cortex are profusely
interconnected by reciprocal connections, which play an important role
in the generation of rhythmic patterns in the brain and in attention, and
may also be involved in top-down effects in perception. By virtue of
bidirectional
connections between the thalamus and every region of the cortex, these
rhythmic patterns sweep regularly and rapidly through the cortex. They,
therefore, provide a possible source for the rapid pacemaker hypothesized
for high-speed inner sequencing. The rate of speed of these thalamocortical
rhythms ranges from twenty to eighty Hz (Hertz: cycles per second) in the
waking state and as slow as from four Hz to less than one Hz in deep sleep.

The hypothalamus (Greek hypo-, cognate
to Latin sub- "under") lies under the thalamus. It plays an
important role
in the regulation of several motivated behaviors. It exerts its effects
in part by regulating the release of hormones from the pituitary gland,
which dangles from it on the ventral surface of the brain. Two other structures
appear on the inferior surface of the hypothalamus  the optic
chiasm
and the mammilary bodies. The optic chiasm is the point at which
the optic nerves from each eye come together. The mammillary bodies are
a pair of spherical hypothalamic nuclei located on the inferior surface
of the hypothalamus, just behing the pituitary gland. The hypothalamus
apparently does not play much of a role in language.